Optical elements such as objective lenses for light pickup devices, spectacle lenses, optical lenses for endoscopes, optical lenses for imaging devices, etc. are provided with optical coatings such as anti-reflection coatings, infrared-absorbing layers, etc. There are, for instance, an anti-reflection coating comprising at least one dielectric layer having a different refractive index from that of a lens for utilizing an interference effect of light (JP 2007-94150 A), and an anti-reflection coating having a rough zinc oxide layer, etc. on the surface (JP 2001-166103 A).
Although an optical element having such optical coating is required to have high scratch resistance, a low-refractive-index, porous layer constituting an outermost layer of the anti-reflection coating and an anti-reflection coating having surface roughness, for instance, have insufficient scratch resistance. When a protective layer is formed to improve the scratch resistance of such anti-reflection coating, the refractive index of the porous layer changes, and the anti-reflecting effect of the roughness decreases, resulting in optical elements with deteriorated performance.
JP 2004-144944 A discloses a spectacle lens provided with an inorganic, hydrophilic, hard layer formed by a metal oxide deposited on a lens substrate from an aqueous solution containing a metal fluoride. It is described that this inorganic, hydrophilic, hard layer comprises silicon oxide, zirconium oxide and titanium oxide, and contains a surfactant in fine recesses on the surface, thereby exhibiting antifogging property and water washability. It is further described that the inorganic, hydrophilic, hard layer has improved scratch resistance. However, the inorganic, hydrophilic, hard layer described in JP 2004-144944 A is formed on a lens substrate surface without an optical coating such as an anti-reflection coating.